1. Field of the Invention
The present invention relates to a zoom lens system, which is suitable for use in a broadcasting television (TV) camera, a video camera, a digital still camera, and a silver-halide camera, and also to an image pickup apparatus including the zoom lens system.
2. Description of the Related Art
In recent years, there have been demanded a zoom lens system having a high zoom ratio and high optical performance for image pickup apparatus such as a television (TV) camera, a silver-halide camera, a digital camera, and a video camera. As the zoom lens system having a high zoom ratio, there is known a four-unit zoom lens system which includes a first lens unit having a positive refractive power, a second lens unit having a negative refractive power for zooming, a third lens unit having a negative refractive power for correcting image plane variation, and a fourth lens unit having a positive refractive power. As to this four-unit zoom lens system, there is known a four-unit zoom lens system having a high zoom ratio, in which an optical material having an anomalous dispersion characteristic is used, so that chromatic aberration is corrected appropriately and a high optical performance is provided (see U.S. Pat. Nos. 5,831,771 and 6,825,990).
A four-unit zoom lens system having a structure described above may support a high zoom ratio relatively easily. In order to obtain high optical performance in this four-unit zoom lens system, it is important to correct longitudinal chromatic aberration at the telephoto end appropriately. It is easy to correct the longitudinal chromatic aberration appropriately if an optical material having an anomalous dispersion characteristic is used. However, it is difficult to correct the chromatic aberration appropriately by simply using a lens made of the optical material having an anomalous dispersion characteristic. In order to obtain high optical performance over the entire zoom range in the four-unit zoom lens system described above, it is an important factor to set appropriately a lens structure of the first lens unit which is fixed during zooming and a material of each lens included in the first lens unit. In this regard, in order to correct the longitudinal chromatic aberration appropriately on the telephoto side, it is important to set an appropriate difference between dispersions of materials of the positive lens and the negative lens in the first lens unit. If this setting is performed inappropriately, it is difficult to correct the longitudinal chromatic aberration appropriately at the telephoto end.